Valve Metal Anode Pellets For Capacitors Formed Using Forced Convection Of Liquid Electrolyte During Anodization

ABSTRACT

A method and apparatus for anodizing a porous valve metal pellet in a flowing liquid electrolyte is described. The apparatus comprises an insulative container comprised of a lower region, a central region including a cavity for holding the pellet, an upper region, and a continuous passageway extending through the lower, central, and upper regions. Lower and upper screens serving as lower and upper electrodes are disposed in the passageway in the lower and upper container regions, respectively. During anodizing, the electrolyte flows through the lower container region including the lower screen, the porous pellet and then the upper container region including the upper screen. The lower and upper screens are at an opposite electrical polarity as the pellet so that a dielectric oxide is formed on the exposed valve metal including interior portions of the pellet that are exposed to the flowing electrolyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/565,766, filed Dec. 1, 2006, now U.S. Pat. No. 7,879,217, whichclaims priority from U.S. Provisional Application Ser. No. 60/742,126,filed Dec. 2, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to anodization of a pellet of avalve metal powder for use as an anode in an electrolytic or anelectrochemical/electrolytic hybrid capacitor. Anodization of the valvemetal pellet or body is performed in an electrolyte that is flowedthrough the valve metal pellet to produce a stable oxide layer with ahigh dielectric constant. A preferred valve metal is tantalum.

2. Description of Related Art

In general, electrolytic capacitors comprise anodes and cathodes thatare physically segregated from each other by a porous separator materialimpregnated with an ionically conductive working electrolyte. Theworking electrolyte is typically composed of water, solvent(s), andsalt(s) of weak inorganic and/or organic acids. The anodes are of avalve metal having its exposed pore surface coated with a film of thecorresponding oxide serving as a dielectric. Valve metals include, butare not limited to, aluminum, tantalum, niobium, titanium, zirconium,hafnium, and alloys thereof. The valve metal can be in any conventionalform including etched foil, sintered powders, or other porousstructures. Anodizing the valve metals in an appropriate anodizingelectrolyte forms a dielectric oxide film thereon. The film thicknessincreases with the anodizing voltage. The desired oxide film thicknessis determined by the capacitor working voltage, operation temperatureand other performance requirements.

It is believed that locally excessive temperatures and insufficientmaterial transport in porous valve metal bodies during anodizing(especially for anodization of high voltage, relatively large, pressedand sintered tantalum powder pellets) causes breakdown or poor anodeelectrical properties. There have been numerous attempts to solve theseproblems by improving the heat and electrolyte transport between thepellets and the bulk electrolytes. Some of the prior art methodsinclude: controlling the anodizing current density; mechanical, sonic,or ultrasonic agitation of the electrolyte; anodizing by combiningcontrol of voltage/current and controlled rest steps (U.S. Pat. No.6,231,993 to Stephenson et al.); and controlled pulses of thevoltage/current (U.S. Pat. No. 6,802,951 to Hossick-Schott). Thesemethods require sophisticated electronics for current/voltage/powercontrol and frequent on/off switches that increase anodizing time.Additionally, it is believed that the eruptive increase incurrent/voltage in the case of pulsed anodizing may cause earlybreakdown and poor oxide quality.

U.S. Patent Application Pub. No. 2006/0196774 to Liu et al. discloses amethod of anodizing valve metals by self-adjusted current and power.U.S. Patent Application Pub. No. 2006/0191796 to Muffoletto et al,discloses a method of anodizing valve metals by controlled power. Thedisclosed methods provide improved anodization of valve metals byapplication of electrical power to the valve metal at specific levelsand in specifically timed on-off sequences. These publications areassigned to the assignee of the present invention and incorporatedherein by reference

Thus, it is known that the conditions for anodizing must be carefullycontrolled in order to provide a suitable oxide layer. In particular,the current applied during formation is generally kept low to avoidelectrical breakdown. Additionally, the current may be turned off orreduced for periods of minutes to several hours during formation, asdescribed in the above patent applications of Liu et al. and Muffolettoet al. The low current and the “off” periods make the formation processvery slow; thirty hours or more may be required to form an anode pelletfor a capacitor. Although there is no general agreement on the precisemechanism by which formation current leads to electrical breakdown, itis thought by some that the current must be kept low in order to avoidoverheating the pellet. Others believe that the formation process islimited by the relatively slow diffusion of electrolyte components orproducts of the electrochemical oxidation.

In any event, the speed of the formation process is limited by theability of conventional means to remove heat and/or “used” electrolytefrom the interior of the pellet. “Conventional” in this sense meansrelying on stirring by means of pumps or stirrers to provide sufficientconvective flow of electrolyte around an anode pellet immersed in anelectrolyte bath. To the extent that there is any increased convectiveheat and/or mass transfer in the electrolyte at the anode pellet, theenhancement likely only occurs at the outer (visible) surface of thepellet, and not at the internal pore surfaces.

Consequently, it has been found that there are greater difficulties inpreparing capacitor anode pellets for high voltage use, and thesedifficulties are increased as the desired thickness of the oxide layeron the pellet surface increases. Among these difficulties are electricalbreakdown which occurs with increasing likelihood at higher formationvoltages, and so-called “gray-out”. “Gray-out” refers to the appearanceof gray or whitish oxide patches on the pellet due to the formation ofcrystalline oxide, which is more “electrically leaky”, i.e., havinglocalized areas of lower electrical resistance, than the desiredamorphous oxide. This may occur when formation is carried out using alow current density with the intention of reducing electricalbreakdowns.

Therefore, there remains a need for an apparatus and a method formanufacturing a valve metal anode such as of the kind typically used inan electrolytic capacitor. Particularly, it is desirable to providevalve metal anodes with dielectric coatings having improved oxidequality and higher breakdown voltages. In that light, the presentinvention teaches an apparatus and a anodization method that reducesprocess time and provides a better quality dielectric oxide. With thepresent invention, the current density used during formation of theoxide layer may be increased and the time required for oxide layerformation decreased. These attributes substantially improve theeconomics of manufacturing valve metal anodes.

Therefore, although this invention is, in principle, applicable to allvalve metal anodes, it is particularly useful for anodizing a highvoltage sintered tantalum pellet for use in an electrolytic capacitor.

SUMMARY OF THE INVENTION

An apparatus for anodizing a valve metal body in a flowing liquidelectrolyte is provided. The apparatus comprises an insulative containercomprised of a lower region, a central region including a cavity forholding the valve metal body, and an upper region. A continuous flowpassageway extends through the lower, central and upper regions. A lowercathode screen is disposed in the passageway in the lower region and anupper cathode screen is disposed in the upper container region. When aliquid electrolyte is delivered into the continuous passageway at thelower region, the electrolyte flows through the lower cathode screen,the cavity of the central region, and then through the upper cathodescreen.

The valve metal body is useful as an anode in an electrolytic capacitorand may be in the form of a relatively thin foil or a pellet.Preferably, the body is a porous pellet formed by sintering a pressedpowder of the valve metal. The pellet may be a cylindrical disc, or ofan irregular shape, such as a crescent shape, as specifically needed foruse in the electrolytic capacitor. Accordingly, the cavity for holdingthe valve metal body is dimensioned to be contiguous with the perimeterof the porous pellet.

The porous pellet may further comprise a lead wire extending therefrom.When the pellet is in the cavity, the lead wire extends to the containerexterior. In a like manner, the lower and upper cathode screens compriseleads extending to the container exterior. The pellet, lead and thecathode sheet leads connect to a power supply. When electrical power isapplied to the cathode sheet leads and the pellet lead while a liquidelectrolyte flows through the container, the pellet is anodized with adielectric oxide formed thereupon. The porous pellet and the first andsecond cathode screens may be made of a valve metal selected from thegroup consisting of tantalum, niobium, aluminum, titanium, and alloysthereof. In operation, the apparatus functions as an electrolytic cellfor anodizing the porous valve metal pellet.

The container may be provided with different configurations for holdingthe cathode screens and the valve metal body in the flowing electrolytestream. In one embodiment, the lower container region comprises a lowercathode retainer, the upper container region comprises an upper cathoderetainer, and the central container region comprises a lower housingportion that is contiguous with an upper housing portion. The cavity forholding the valve metal body is formed between the lower and upperhousing portions. The lower cathode screen is disposed between the lowercathode retainer and the lower housing portion. The upper cathode screenis disposed between the upper cathode retainer and the upper housingportion. The retainers and housing portions are made of an insulatingmaterial that prevents an electrical short circuit between the cathodescreens and the valve metal body when a voltage is applied to them. Onepreferred insulative material is polypropylene.

In another embodiment, the lower container region comprises a lowerretainer housing joined to a lower retainer plate, the upper containerregion comprises an upper retainer housing joined to an upper retainerplate, and the central container region comprises a lower housingportion that is contiguous with an upper housing portion. The cavity forholding the valve metal body is formed between the lower and upperhousing portions. In this configuration, the lower cathode screen isdisposed between the lower retainer housing and the lower retainerplate, and the upper cathode screen is disposed between the upperretainer housing and the upper retainer plate. The apparatus may furtherinclude a first filter mesh disposed in the continuous passagewaybetween the lower cathode screen and the valve metal body cavity and asecond filter mesh disposed in the passageway between the upper cathodescreen and the cavity.

In another embodiment, only a single cathode screen is provided in theapparatus. In this configuration, the apparatus comprises a containercomprised of a lower region, a central region including a cavity forholding the valve metal body, an upper region, and a continuouspassageway extending through the lower, central and upper regionsincluding the central region cavity. A cathode screen is disposed in thecontinuous container passageway. When a liquid electrolyte is deliveredinto the continuous passageway at the lower region, the electrolyteflows from the lower region, through the central region cavity and thecathode screen, and through the upper region. The cathode screen may belocated in either the lower region or the upper region. However,superior oxide films are formed on the valve metal body when cathodescreens are located both upstream and downstream of the body duringanodization.

Also according to the present invention, a method for anodizing a valvemetal body is described. The method comprises securing the valve metalbody in a cavity within a container comprised of a lower region, acentral region including the valve metal body cavity, and an upperregion. A continuous passageway extends through the regions. At leastone cathode screen is disposed in the continuous container passageway. Aliquid electrolyte is flowed through the passageway from the lowerregion including the perforated cathode screen, through the cavity andover and through the valve metal body. The cathode screen and the valvemetal body are connected to opposite polarity terminals of an electricalpower supply, thereby causing an electrical current to flow through theelectrolyte between the cathode screen and the valve metal body. Thisforms an oxide layer on the exposed surfaces of the valve metal body.

Also according to the present invention, an anode for a capacitor isprovided comprising tantalum powder in the form of a porous body. Thetantalum is characterized as having been anodized in a liquidelectrolyte flowing through the body to form an oxide layer on theexposed surfaces thereof. Additionally, a capacitor is providedcomprising a porous tantalum body characterized as having been anodizedin a flowing liquid electrolyte to form an oxide layer on the exposedsurfaces thereof, a cathode, a separator segregating the anode from thecathode, and a working electrolyte.

The foregoing and additional objects, advantages, and characterizingfeatures of the present invention will become increasingly more apparentupon a reading of the following detailed description together with theincluded drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described by reference to the followingdrawings, in which like numerals refer to like elements, and in which:

FIG. 1 is a schematic illustration of an apparatus for anodizing a valvemetal pellet immersed in a flowing stream of electrolyte;

FIG. 2 is a side cross-sectional view of one embodiment of a containercomprising the anodizing apparatus shown in FIG. 1;

FIG. 3 is an exploded cross-sectional view of the container of FIG. 2;

FIG. 4 is a perspective view of another embodiment of a container foranodizing a valve metal pellet immersed in a flowing stream ofelectrolyte, wherein the valve metal pellet may have an irregular shape;

FIG. 5 is an exploded view of the container of FIG. 4; and

FIG. 6 is a cross-sectional view taken along the line 6-6 of FIG. 4.

The present invention will be described in connection with preferredembodiments, however, it will be understood that there is no intent tolimit the invention to the embodiments described. On the contrary, theintent is to cover all alternatives, modifications, and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Porous valve metal pellets are preferred in electrolytic capacitors usedin implantable medical devices such as cardioverter defibrillators. Theanodizing apparatus and method of the present invention are particularlyuseful for anodizing large and high voltage sintered powder anodes suchas those used in tantalum electrolytic capacitors powering implantablemedical devices. In that respect, the apparatus and method of thepresent invention are directed to forming valve metal bodies into anodesby providing the bodies with dielectric coatings having improved oxidequality and high breakdown voltages. The valve metals to be made intoanodes can be in any conventional form, such as etched foil, sinteredpowders, or other porous structures. The following description will beprovided with reference to a particular valve metal form made bysintering a pressed powder pellet. It is to be understood, however, thatthe apparatus and methods of the present invention are applicable toother valve metal objects such as foils, sheets, and solid pellets in avariety of shapes and sizes. Any of these may be anodized, provided thatthe present apparatus is configured to provide a flow of electrolyteover and through the body during anodization, as will be describedherein.

In the anodization apparatus and method of the present invention, therelative motion of the electrolyte with respect to the anode pellet isincreased over that of conventional anodization methods in which theanode pellet is immersed in a quiescent or stirred electrolyte bath.This is accomplished by taking advantage of the porous structure of theanode pellet. Liquid electrolyte is forced through the pores of thepellet under pressure. This forced internal convection providesincreased heat and mass transfer in the electrolyte within the interiorof the pellet.

It is noted that in the following description, the apparatus isdescribed with reference to a “lower” region and an “upper” region, withupper and lower being in the conventional sense with respect to gravity.The container of the apparatus is shown and described in this mannerwith a preferred orientation with respect to gravity, and the preferreddirection of electrolyte flow through the container being upward. Thisis because in some circumstances, a gas may be evolved at the cathode(s)and an upward electrolyte flow ensures continuous purging of any evolvedgas. However, it is to be understood that the present apparatus may beoperated with orientations other than shown such as a horizontalorientation while still achieving satisfactory results. Accordingly, theterms “lower” and “upper” are not intended to be limiting with respectto orientation of the apparatus, but rather referring to the relativepositions of the various components with respect to each other.

Additionally, the electrolyte used in the anodization apparatus andmethods described herein are generally referred to as a “liquid”electrolyte. This means that the electrolyte flowed through theapparatus when the anodization process begins is a single phasehomogeneous liquid. However, in the electrochemical reaction at thecathode(s), hydrogen gas may be evolved and, consequently, the flowingelectrolyte within the container may become a two-phase fluid includingbubbles of evolved gas. Therefore, as used herein, the term “liquidelectrolyte” includes, but is not limited to, a homogeneous single phaseliquid electrolyte, a single phase liquid electrolyte containingdissolved gases or a concentration gradient of ionic species, and aliquid phase electrolyte with gas bubbles distributed therein.

Referring now to the drawings, FIGS. 1 to 3 show an apparatus 10comprises an insulative container 100, which functions as anelectrolytic cell in the anodization process. During the process, aliquid electrolyte is flowed through container 100 and through a porousvalve metal pellet 50 held therein. Apparatus 10 also includes areservoir 20 for containing the liquid electrolyte, a pump 30 andvarious liquid conduits for delivering electrolyte from the reservoir 20through container 100 and back to the reservoir. Apparatus 10 furthercomprises a power supply 40 for delivering an electrical current throughone or more cathodes in the container 100, and through the valve metalpellet 50 therein, which functions as an anode in the circuit.

Container 100 is comprised of an exterior, a lower region 110, a centralregion 140 including a cavity 150 for holding the porous pellet, and anupper region 170. A continuous passageway 102 within the interior ofcontainer 100 extends through the lower region 110, the central region140 including its cavity 150, and the upper region 170.

A lower screen 130 is disposed in the passageway 102 in the lowercontainer region 110. The lower screen 130 comprises a sheet ofelectrically conductive material with at least one perforation 132, andpreferably many perforations, in order to permit electrolyte flowtherethrough. In like manner, an upper screen 190 is disposed inpassageway 102 in the upper container region 170. The upper screen 190also comprises a sheet of electrically conductive material with one ormore perforations 192 therethrough. Screens 130, 190 may be made fromany suitable electrically conducting material which is perforated topermit the flow of electrolyte. Examples include a valve metal,preferably the same as that of the valve metal pellet 50, carbon,platinum, nickel, or other conductive material that is sufficientlystable under operating conditions. The screen may be made from acontinuous sheet with perforations punched, cut, or drilledtherethrough, or from a woven wire mesh with the perforations 132, 192formed by the interstices between the wires. A thin sintered wafer ofvalve metal material is also suitable.

When liquid electrolyte is delivered into the continuous passageway 102at the lower region 110, the electrolyte flows through the lower screen130, the cavity 150 of the central region 140 and the porous pellet 50held therein, and through the upper screen 190. As described previously,other valve metal forms may be substituted for porous pellet 50.

An example of an effective liquid electrolyte for the anodizing processis disclosed in commonly assigned U.S. Pat. No. 6,231,993 to Stephensonet al, and comprises an aqueous solution of ethylene glycol orpolyethylene glycol and H₃PO₄. Another exemplary electrolyte isdescribed in U.S. application Ser. No. 11/559,968, filed Nov. 15, 2006,and comprises about 80 volume percent polyethylene glycol (PEG400) witha minor volume percent amount of H₃PO₄ and remainder de-ionized water,and has a resistivity of about 1,000 ohm-cm to about 30,000 ohm-cm at40° C.

The porous pellet 50 to be anodized is preferably formed by sintering apressed body of valve metal powder. Suitable valve metals and methodsfor sintering them are disclosed in U.S. Pat. No. 6,695,510 to Liu etal. This patent is assigned to the assignee of the present invention andincorporated herein by reference. The porous pellet 50 is formed as athin disc, having an outer edge 52 defining a perimeter thereof. Thepellet 50 may be cylindrical as shown in FIG. 2, or of an irregularshape determined specifically for use in a particular electrolyticcapacitor. Accordingly, the cavity for holding the valve metal body isdimensioned to be contiguous with the perimeter of the porous pellet. Anexample of such an irregularly shaped porous pellet is shown as pellet51 in FIG. 5, which is held in a correspondingly shaped cavity formedbetween a lower housing portion 242 and an upper housing portion 262.

Referring again to FIGS. 2 and 3, the porous pellet 50 may furthercomprise a lead wire 54 extending therefrom. When the pellet 50 iscontained within the container cavity 150, the lead wire 54 extendsoutside the container 100. The central container region 140 is providedwith a port 154 for routing the lead wire 54 from the cavity 150 to thecontainer exterior. In like manner, the lower screen 130 comprises alower lead 134 extending to the container exterior, and the upper screen190 comprises an upper lead 194 extending to the container exterior.Ports 114 and 174 are provided in lower and upper sections 110 and 170for routing the respective lead wires 134 and 194 to the containerexterior. If necessary, seals or a small amount of semi-solid sealingmaterial (neither shown) may be provided around the lead wires 54, 134and 194 or at the screen 130, 190 perimeters to prevent electrolyte fromleaking out of the container 100.

The leads 54, 134 and 194 connect the power supply 40 to the respectivepellet 50 and cathode screens 130, 190. Referring to FIG. 1, thenegative terminal 42 of the power supply 40 is connected to the leads134, 194 of the screens 130, 190 serving as negative electrodes orcathodes and the positive terminal 44 is connected to the lead wire 54of the pellet 50 serving as a positive electrode or anode. That way,when electrical power is applied to the screen leads 134, 194 and thepellet lead 54 while a liquid electrolyte flows therethrough, thescreens 130, 190 are at a negative polarity with respect to the positivepolarity of the pellet 50. This results in the exposed surfacesincluding the interior porous surfaces of the pellet 50 being anodizedwith an oxide of the valve metal formed thereupon. In operation, theapparatus 10 functions as an electrolytic cell when anodizing the porouspellet 50. As used in this instance, the term “surface” of the pellet ismeant to indicate the entire surface at the solid-liquid interface,including the external (visible) surface as well as the interstitialpore surfaces of the porous pellet 50.

The container may be provided with different configurations for holdingthe cathode screens and the porous pellet or other valve metal bodies inthe flowing electrolyte stream. In one embodiment depicted in FIGS. 2and 3, the lower container region 110 comprises a lower cathode retainer120, the upper region 170 comprises an upper cathode retainer 180, andthe central region 140 comprises a lower housing portion 142 that iscontiguous with an upper housing portion 162. The cavity 150 for holdingthe porous pellet 50 is formed between the lower housing portion 142 andthe upper housing portion 162. Countersinks 144 and 164 are formed inthe lower housing portion 142 and the upper housing portion 162,respectively. That way, when the housing portions are assembled, thepellet cavity 150 is formed therebetween.

Lower cathode screen 130 is disposed between the lower cathode retainer120 and the lower housing portion 142, and the upper cathode screen 190is disposed between the upper cathode retainer 180 and the upper housingportion 162. The retainers 120, 180 and housing portions 142, 162 aremade of an insulating material that prevents an electrical short circuitbetween the cathode screens 130, 190 and the porous pellet 50 when avoltage is applied to them. One preferred insulative material ispolypropylene. Natural polypropylene is preferred because it contains noadditives that can leach out and contaminate the electrolyte, andpossibly cause impurities and defects to form in the oxide layer. Otherpolymers or ceramic materials may be suitable. However, the operativerequirement is that the material be electrically insulative, resistantto chemical degradation by the electrolyte, non-reactive in theanodization process, and contains no leachates that may contaminate theelectrolyte.

In the embodiment depicted in FIGS. 2 and 3, retainers 120, 180 andhousing portions 142, 162 are provided with matched threads forfastening them to each other. Seals (not shown) and/or thread sealantsmay also be used to prevent leakage of the electrolyte from thecontainer. It will be apparent that retainers 120, 180 and housingportions 142, 162 may be secured and sealed to each other by other meansknown in the fluid handling arts.

Another embodiment of a suitable container for the anodizing process isshown in FIGS. 4 to 6. This container is advantageous in the anodizationof irregularly shaped anode pellets. Container 200 is comprised of alower region 210, a central region 240, and an upper region 270 forminga continuous passageway for liquid electrolyte flow, as indicated byarrows 297, 298 and 299 (FIG. 6).

The lower region 210 comprises a lower retainer housing 212 joined to alower retainer plate 222 by suitable fastening means such as screws 214(FIG. 5). A lower cathode screen 230 formed with perforations 232therethrough is disposed between the lower retainer housing 212 and thelower retainer plate 222. A polymeric O-ring seal 211 is providedtherebetween surrounding the cathode screen 230. In a like manner, theupper region 270 comprises an upper retainer plate 282 joined to anupper retainer housing 272. An upper cathode screen 290 withperforations 292 surrounded is disposed therebetween. A polymeric O-ringseal 291 surrounds the cathode screen 290 between the upper retainerhousing 272 and the upper retainer plate 282. A polymeric O-ring seal221 is provided between the lower retainer plate 222 and the lowerhousing portion 242 and another polymeric O-ring seal 261 is providedbetween the upper housing portion 262 and the upper retainer plate 282.

The central region 240 of the container 200 comprises a lower housingportion 242 that is contiguous with an upper housing portion 262. Thelower housing portion 242 comprises a passageway 243 of a restrictedcross-section with respect to the passageway through the lower retainerhousing 212 and the lower retainer plate 222 that terminates at anoutwardly extending beveled edge 244. Similarly, the upper housingportion 262 comprises a passageway 263 of a restricted cross-sectionwith respect to the passageway through the upper retainer plate 282 andthe upper retainer housing 272 that terminates at an outwardly extendingbeveled edge 264. The restricted cross-sections provide for increasedfluid flow through this area for more effective movement of theelectrolyte over and through the porous valve metal pellet.

The cavity for holding porous pellet 51 is formed between the lowerhousing portion 242 and the upper housing portion 262 where the bevelededges 244 and 264 meet. During anodization, the porous pellet 51 is heldwithin this cavity surrounded by the sealed contact of its perimeteredge 53 with the beveled edges 244, 264. Polymeric O-ring seals 241A and2418 are contiguous with each other and surround the pellet 51 betweenthe lower and upper housing portions 242, 262. The porous pellet 51 isformed with a typical crescent shape used in electrolytic capacitors.

Porous pellet 51 further comprises a lead wire 55 extending therefrom.When the pellet 51 is confined within the container cavity where thepassageways 243, 263 meet, the lead wire 55 extends between the seals241A, 2418 to the exterior thereof. This is because the lower housingportion 242 is provided with a channel groove 246 (FIG. 5) and upperhousing portion 262 is provided with a corresponding channel (not shown)that together form a port for routing the lead wire 55 from the cavityto the exterior of the container 200.

The lower cathode screen 230 further comprises a lead 234, preferablyformed as a tab, extending to the exterior of the container 200.Similarly, the upper cathode screen 290 comprises a lead 294 extendingto the container exterior. During the anodization process, leads 55, 234and 294 are connected to the power supply 40 (FIG. 1) as previouslydescribed for container 100 of FIGS. 2 and 3.

The lower region 110, central region 140, and upper region 170 ofcontainer 200 are held together during anodization by tie rod assemblies202, 204 and 206. It will be apparent that other fastening means wouldbe suitable for holding the container components.

The container 200 may further include a first filter mesh (not shown)disposed in the continuous passageway between the lower cathode screen230 and the cavity for holding the porous pellet 51. A second filtermesh (not shown) is disposed in the continuous passageway between theupper cathode screen 290 and the cavity. The filter meshes prevent solidparticulate impurities from being entrained in the flowing electrolyte.Such solid impurities could adversely affect the dielectric oxide layerby forming defects that could cause current leakage or oxide breakdown.

The configuration of container 200 is advantageous in the manufacture ofcapacitor anode pellets for several reasons. Container 200 is configuredfor manufacturing of anode pellets of a variety of shapes with minimaltooling costs. Anode pellets of various shapes and sizes can be anodizedby changing out only the lower and upper housing portions 242, 262 ofthe central region 240 with the desired cavity to match the particularanode pellet shape. No other special components are required. Using thetie rod assemblies 202 to 206, container 200 is easily assembled with aporous pellet held therein, and disassembled to remove the anodizedpellet after processing. Lower region 210 and upper region 270 aremaintained as unitary subassemblies that do not need to be dismantledduring pellet changeover. The various components of container 200 aremade of suitable insulative materials, as described previously forcontainer 100 of FIGS. 2 and 3.

The containers 100 and 200 of FIGS. 2, 3 and 4 to 6 may be assembled andoperated with only a single cathode screen therein. The cathode screenmay be located either in the lower region or the upper region of thecontainer. However, the single cathode configuration is not preferred.This is because superior oxide films are formed on the valve metal bodywhen cathode screens are located both upstream and downstream of thevalve metal body during anodizing. It is believed that during theanodization of a sintered tantalum pellet, the use of two cathodescreens produces a more uniform current distribution in the interstitialspace inside the pellets 50, 51.

The containers 100 and 200, and the apparatus 10 of the presentinvention are particularly useful for anodizing large and high voltagesintered powder anode pellet, such as those used in tantalumelectrolytic capacitors. A method to anodize a porous pellet includesthe steps of securing the pellet in a cavity within a container 100 or200, causing a liquid electrolyte to flow through the passageway withinthe container, through at least one cathode screen, and over and throughthe porous pellet, connecting the at least one cathode screen and theporous pellet to an opposite polarity terminal of the electrical powersupply, thereby causing an electrical current to flow through the liquidelectrolyte between the cathode screen and the anode pellet, and formingan oxide layer on the surface of the porous pellet.

For the sake of manufacturability and efficiency, however, it isdesirable to reduce the size and cost of the formation equipment. Thismay be accomplished in a variety of ways, for example, several cavitiesmay be used for simultaneous formation of multiple pellets, in which theseveral cavities may be fed from a common reservoir of liquidelectrolyte. The cavities are preferably arranged in parallel withrespect to the liquid flow so that each cavity receives freshelectrolyte from the common reservoir. The cavities may be in individualcontainers or set into a common plate. Thus, a single pump may be usedto supply electrolyte to several forming valve metal pellets, or eachcavity may be supplied by its own pump. A similar principle applies tothe electrical power supply. Each set of anode and cathode screens mayhave its own power supply, or a single power supply may be used toprovide power to several formations. Different protocols for theapplication of electrical power to the cathode screens and anode pelletmay be used during anodization. Electrical current may be deliveredthrough the electrodes continuously, or intermittently, with the currentbeing maintained constant, or the current being varied. Variousanodization protocols are described in the aforementioned U.S. Patent.Application Pub. Nos. 2006/0196774 to Liu et al. and 2006/0191796 toMuffoletto et al.

Example I

Tantalum anode pellets were prepared according to the present inventionas follows. Cylindrical pellets were obtained from the Tantalum PelletCompany. They were sintered at 1,630° C. for 20 minutes and had nominaldimensions of 0.07 inches thick and 1.02 inches in diameter, and weighedabout 6.7 grams. Each pellet was placed in a container of the presentinvention, and an electrolyte was delivered through the pellet. Theelectrolyte consisted of 77% polyethylene glycol (PEG400), 7.5%phosphoric acid, and 15.5% water at 40° C., by vol. Two pellets wereformed by application of electrical power to the cathode and pelletleads according to the schedule in Table 1.

TABLE 1 Current Ending Voltage Hours Held at Hours (milliamps) (V)Voltage Off 134 150 1 1 87 190 0 1 67 225 1 1 40 255 0 1 40 280 1 1 20310 0 1 20 330 1 1 20 355 1 1 20 375 1 1 20 390 1 1 20 410 1 1 20 430 11 20 445 1 0 20 445 1 0

After formation, the pellets were rinsed for 6 hours, dried, and thenheat treated in air at 420° C. for 30 minutes before reformation. Thereformation step consisted of holding the pellet at 445 volts for onehour in the formation electrolyte.

After reformation, the DC current leakage (DCL) of the pellets wasmeasured. The value was obtained after holding for 5 minutes at 415volts in the formation electrolyte at 40° C. The values for DCL obtainedwere 39 and 40 microamperes. It is common practice to express theleakage in units of nanoamps/(μF·V_(w)) where μF is the capacitance inmicrofarad at 120 Hz and V_(w) is the working voltage, in this case 415volts. Values of about 1.0 nA/(μF·V_(w)) were obtained for thesepellets, which is a satisfactory value.

Example II

Another pellet was formed and tested in the same manner as described inExample I except that the formation protocol in Table 1 was changed. The“hours held at voltage” in column 3 of Table 1 was reduced to 0.1 hours.The DC leakage was essentially identical to the values obtained for theother pellets; however, the formation time was reduced by about 12hours. This demonstrates the advantage of the present invention inmaking suitable capacitor anode pellets at a significantly higherthroughput.

Example III

For a further comparison of formation time and properties of valve metalpellets made according to the conventional procedure and according tothe method described herein, 20 identical “D” shape pellets obtainedfrom Tantalum Pellet Company were formed to 420 volts. The pellets eachhad a weight of about 8.2 grams and a green thickness of 0.072 inches.They were sintered at 1,650° C. for 20 minutes. Ten of the pellets wereformed using a conventional tank formation procedure according to theprotocol shown in Table 2.

TABLE 2 Current Voltage Hold Time Off Time (mA) (V) (hours) (Hours)108.9 75 0 1 54.4 150 0 1 43.0 190 0 1 36.3 225 0 1 32.0 255 0 1 29.2280 0 1 26.3 310 0 1 24.7 330 0 1 23.0 355 0 1 21.8 275 0 1 20.9 390 0 119.9 410 0 1 19.4 420 1 0

The other ten pellets were formed in containers of the presentinvention. Two containers were used simultaneously with a commonelectrolyte reservoir. A separate power supply and pump was used foreach container. The flow rate to each pellet was controlled to about 300millimeters/minute by means of individual peristaltic pumps. Thesepellets were formed according to the protocol in Table 3.

TABLE 3 Current Voltage Hold Time Off Time (mA) (V) (hours) (Hours) 163150 0 0.1 129 190 0 0.1 109 225 0 0.1 94 260 0 0.1 83 296 0 0.1 74 330 00.1 56 365 0 0.1 41 400 0 0.1 29 420 2 0.1

The median formation times for the two sets of pellets were 62 hoursusing the conventional procedure and 25 hours using the apparatus of thepresent invention. Median nA/uF·V values were 0.64 and 0.67. The greatlyreduced formation time for the pellets of the present invention was madepossible by the higher current densities used, which provided improvedelectrolyte flow and electrolyte cooling in the pellets' interstitialpore space.

Thus, it should be apparent to those skilled in the art that the claimedapparatus and method offers the following advantages over the priorart: 1) increased heat and mass transfer in the electrolyte within theinterior of the pellet throughout the course of anodizing, therebyavoiding excessive temperature and accumulation of spent electrolyte atthe valve metal structure; 2) a relatively short anodizing time; and 3)simplified anodizing electronics and equipment resulting in a low costanodization protocol. The claimed anodization protocol also results inimproved anode electrical properties including lower DC leakage, morestable shelf life, improved charge/discharge energy efficiency, andimproved stability during operation life. These properties are stronglydesired for critical applications such as for the anode of a capacitorpowering an implantable cardioverter defibrillator.

It is, therefore, apparent that there has been provided, in accordancewith the present invention, a method and apparatus for anodizing a valvemetal body in a flowing liquid electrolyte. While this invention hasbeen described in conjunction with preferred embodiments thereof, it isevident that many alternatives, modifications, and variations will beapparent to those skilled in the art. Accordingly, it is intended toembrace all such alternatives, modifications and variations that fallwithin the spirit and broad scope of the appended claims.

1. An apparatus for anodizing a valve metal body in a liquidelectrolyte, the apparatus comprising: a) a container having an exteriorproviding a continuous passageway extending along and through a lowercontainer region and a central container region including at least onecavity for holding a valve metal body; b) a valve metal body containedin the cavity in fluid flow communication with the container passageway;c) a lower electrode disposed in the passageway in the lower containerregion; and d) wherein the liquid electrolyte is flowable past the lowerelectrode and the valve metal body being maintained at an oppositepolarity with respect to each other to thereby form a dielectric oxideon the valve metal body.
 2. The apparatus of claim 1 wherein the lowerelectrode is at a negative polarity serving as a cathode and the valvemetal body is at a positive polarity serving as an anode.
 3. Theapparatus of claim 1 wherein the container further comprises an uppercontainer region having an upper electrode disposed in the passagewaytherein, the upper electrode being maintained at the same polarity asthe lower electrode.
 4. The apparatus of claim 1 wherein the lowerelectrode is a perforated screen that allows the electrolyte to flowtherethrough.
 5. The apparatus of claim 1 wherein the valve metal bodyfurther comprises a lead wire and the central container region comprisesa port for routing the lead wire from the cavity to the containerexterior.
 6. The apparatus of claim 1 wherein the lower electrodecomprise a lead extending to the container exterior.
 7. The apparatus ofclaim 1 further comprising a power supply having a first terminalelectrically connected to the lower electrode and a second, oppositepolarity terminal electrically connected to the valve metal body.
 8. Theapparatus of claim 1 wherein the lower electrode is selected from thegroup consisting of carbon, platinum, nickel, tantalum, niobium,aluminum, titanium, and alloys thereof.
 9. The apparatus of claim 1wherein the lower electrode is a screen of a valve metal that is eitherthe same or different as that of the valve metal body.
 10. The apparatusof claim 3 wherein the lower container region comprises a lowerelectrode retainer, the upper container region comprises an upperelectrode retainer, the central container region comprises a lowerhousing portion that is contiguous with an upper housing portion, andthe cavity for holding the valve metal body is formed between the lowerhousing portion and the upper housing portion.
 11. The apparatus ofclaim 10 wherein the lower electrode is a lower screen disposed betweenthe lower electrode retainer and the lower housing portion, and theupper electrode is an upper screen disposed between the upper electroderetainer and the upper housing portion.
 12. The apparatus of claim 1wherein the container is of an electrically insulative material.
 13. Theapparatus of claim 3 wherein the lower container region comprises alower retainer housing joined to a lower retainer plate, the uppercontainer region comprises an upper retainer housing joined to an upperretainer plate, the central container region comprises a lower housingportion that is contiguous with an upper housing portion, and the cavityfor holding the valve metal body is between the lower housing portionand the upper housing portion.
 14. The apparatus of claim 13 wherein thelower electrode is an electrically conductive screen disposed betweenthe lower retainer housing and the lower retainer plate, and the upperelectrode is an electrically conductive screen disposed between theupper retainer housing and the upper retainer plate.
 15. The apparatusof claim 1 wherein the valve metal body is a porous pellet having aperimeter and wherein a cavity sidewall seals against the valve metalbody perimeter so that the electrolyte flows through the valve metalbody.
 16. The apparatus of claim 1 further comprising at least onefilter disposed in the continuous passageway to entrain impurities inthe electrolyte.
 17. The apparatus of claim 1 wherein the container hastwo or more cavities arranged in parallel and fed with fresh electrolytefrom a common electrolyte reservoir, each cavity holding a valve metalbody.
 18. The apparatus of claim 1 wherein the two or more cavities areset into a common plate.
 19. The apparatus of claim 18 wherein the valvemetal bodies held in the two or more cavities are each at a positivepolarity serving as an anode with respect to at least one negativepolarity electrode serving as a cathode and connected to a common powersupply.
 20. The apparatus of claim 18 wherein the valve metal bodiesheld in the two or more cavities are each at a positive polarity servingas an anode with respect to at least one negative polarity electrodeserving as a cathode and connected to a power supply dedicated to eachvalve metal body.
 21. An apparatus for anodizing a valve metal body in aliquid electrolyte, the apparatus comprising: a) a container of anelectrically insulative material having an exterior providing acontinuous passageway extending along and through a lower containerregion, a first central container region including a first cavity forholding a first valve metal body, and an upper container region; b) alower electrode disposed in the passageway in the lower containerregion; c) an upper electrode disposed in the passageway in the uppercontainer region; d) wherein the first central container region isdetachable from the lower and upper container regions and replaceable bya second central container region having a second cavity for holding asecond valve metal body that is shaped differently than the first valvemetal body; and e) wherein the liquid electrolyte is flowable past thelower container region including the lower electrode, the first orsecond central container regions including the first or second cavitiesholding the respective first or second valve metal bodies, and the uppercontainer region including the upper electrode with the upper and lowerelectrodes being maintained at an opposite polarity as the first or thesecond valve metal body.
 22. The apparatus of claim 21 wherein a cavitysidewall of both the first and second central regions is contiguous witha perimeter of the first or the second valve metal body, as the case maybe.
 23. The apparatus of claim 21 wherein the upper and lower electrodesare perforated screens that allow the electrolyte to flow therethrough.24. The apparatus of claim 21 wherein the first and second valve metalbodies comprise first and second leads connectable to a first terminalof a power supply and the lower and upper electrodes comprise respectiveleads electrically connectable to an opposite polarity of the powersupply.
 25. The apparatus of claim 21 wherein the lower and upperelectrodes are screens of a valve metal selected from the groupconsisting of tantalum, niobium, aluminum, titanium, and alloys thereofthat is the same as that of the first the second valve metal body. 26.An anode for a capacitor, the anode comprising: a valve metal powder inthe form of a body providing porosity characterized as having beenanodized in an electrolyte flowing through the body from one major faceto an opposite major face thereof to form a dielectric oxide on theporous valve metal body.
 27. The anode of claim 26 wherein the valvemetal is selected from the group consisting of aluminum, tantalum,niobium, titanium, zirconium, hafnium, and alloys thereof.
 28. The anodeof claim 26 wherein the valve metal body has a lead wire extendingtherefrom.
 29. A capacitor comprising: a) an anode comprising a valvemetal powder provided in the form of a porous body having a surface,wherein the valve metal is characterized as having been anodized in aflowing liquid electrolyte to form an oxide layer of the valve metal onthe surface of the porous body; b) a cathode; c) a separator segregatingthe anode from the cathode; and d) a working electrolyte.
 30. Thecapacitor of claim 29 wherein the valve metal is selected from the groupconsisting of aluminum, tantalum, niobium, titanium, zirconium, hafnium,and alloys thereof.